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Question involving capacitance.

  1. Feb 23, 2014 #1
    A circuit uses a neon lamp. The neon lamp has a threshold voltage V0 for conduction, because no current flows until the neon gas in the tube is ionized by a sufficiently strong electric field. Once the threshold voltage is exceeded, the lamp has negligible resistance.

    The capacitor stores electrical energy, which can be released to flash the lamp. Assume that C = 4.00×10^-8 F, R = 1.4200×10^6 Ω, V0 = 90.00 V and ε = 110.0 V.

    The 110 V is connected to R and C in series and the neon lamp is in parallel with C.

    Question: A typical light bulb emits at 100W. What is the ratio of energy emitted by the neon flash to the light bulb (Assuming the light bulb is turned on for the same amount of time as the neon bulb)?

    Through my previous calculations I determined that the amount of energy released by a flash of light is 1.62x10^-4J. How do I attain time to answer the above question?
  2. jcsd
  3. Feb 23, 2014 #2


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    Hello Allen, and welcome to PF. Please do use the template. What previous calculations led you to 1.62x10-4 J ? (don't bother, I can tell).
    Now, since the wording 'negligible resistance' doesn't help you in finding a discharge time for the capacitor, perhaps you are supposed to look at the charging time. That's where the 2.) relevant equations of the template comes in so nicely: do you have something at hand to calculate how long it takes to charge the capacitor from 0 ('negligible resistance' !) to V0 ?
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